1. Field of the Invention
This invention relates to a process for the synthesis of ortho-sulfobenzoic acid anhydride which comprises heating a 2-sulfohalide benzoate in the presence of a catalytic amount of a Friedel-Crafts catalyst.
Ortho-sulfobenzoic acid anhydride is a useful chemical intermediate in the manufacture of dyes, saccharin, and sulfonephthalein dyes and indicators. It is also useful as a polymerization inhibitor.
2. Description of the Prior Art
Several methods for the synthesis of ortho-sulfobenzoic acid anhydride have been described in the prior art. For example, U.S. Pat. Nos. 4,042,600 and 4,259,499 teach the preparation of ortho-sulfobenzoic acid anhydride by the pyrolysis of 2-sulfohalide benzoates. These disclosures, however, do not indicate the use of catalysts of any kind, and the procedure taught in these patents does not give high yields at reasonable processing temperatures.
H. T. Clark and E. E. Dreger, Org. Syn. Coll. Vol. I, Second Edition, page 495 (1941), teach the synthesis of ortho-sulfobenzoic acid anhydride by the reaction of acid ammonium ortho-sulfobenzoate with excess thionyl chloride, usually in an aromatic solvent. This reaction does not proceed in high yield, and has the additional disadvantage that sulfur dioxide and hydrochloric acid gas are given off in the course of the reaction.
Belgian Patent No. 629,100 teaches the ring-closure reaction of ortho-sulfobenzoic acid.